There are hundreds (perhaps over a thousand) airplanes that will be grounded on January 1, 2014.
The FAA won't have completed the paperwork for a large block of airplanes that are queued for re-registration.
You will recall that the FAA mandated re-registration of all civil aircraft started in 2010 under the authority of 49 USC § 44101 et. al. The FAA, over objections, imposed 14 CFR § 47.40 that requires a 3 year re-registration. So far, it's a mandate making GA more of a hassle—but manageable—only if FAA does its job (always a questionable proposition).
(H/T Mark Sletten)